Light induced refractive index changes in the core of an optical fiber were first observed in Ge-doped optical fibers, as described in Applied Physics Letters 32(10), pp 647-649, 1978 "Photosensitivity in Optical Fiber Waveguides: Application to Filter Fabrication" by K. O. Hill et al, and in U.S. Pat. No. 4,474,427 issued Oct. 2, 1984. To produce the changes, light having a wavelength in the visible region was launched into the core of a Ge-doped fiber strand. The light was reflected from the end of the fiber. The forward propagating light interfered with the backward propagating light to form a standing wave pattern with a period corresponding to half the wavelength of the writing light. Through a photosensitive effect in the fiber, a refractive index grating having this period was written in the core of the fiber.
With the above technique, gratings could only be fabricated with reflected light having wavelengths close to that of the writing light. An improvement to that process was described in U.S. Pat. No. 4,807,950 issued Feb. 28, 1989. In that patent the gratings were produced in the fiber by illuminating the fiber from the side with coherent ultraviolet radiation of 245 nm wavelength. By using two light beams, an interference pattern is set up along the length of the fiber. The period of the pattern could be controlled by controlling the angles of the interfering beams. Therefore index gratings could be written in the fiber which would reflect light at longer wavelengths.
A fiber optic converter was created by impressing periodic microbends in the fiber along its length, at a correct period between bends to induce mode coupling. The microbends were usually induced by laying a flat metal block with fine parallel grooves over the fiber. Another approach was to launch into a two-mode fiber light with high power in both modes. The interference pattern of the two propagating modes induced in the fiber core a permanent index of diffraction grating of the correct period for mode coupling. If light is now launched into one of the modes of the two mode fiber, the index grating couples it with the other mode.
Internal writing with core-guided light has the disadvantage that it usually limits the resonance of the filters that are produced thereby to the writing wavelength and perhaps to a few related wavelengths. This limitation can be overcome by resorting to external writing. An external point-by-point writing technique as described in copending U.S. patent application Ser. No. 656,462 filed Feb. 19, 1991 and assigned to the same assignee is useful for the fabrication of very long periodic and a periodic grating structures, the order of meters, that can act as intermode converters. The technique is termed an external writing technique because the light used for writing the index perturbation point-by-point irradiates the optical fiber from the side. In contrast to internal techniques, external writing techniques have the advantage that mode converters can be fabricated in standard optical fiber and operated at wavelengths used in optical communications systems.